Like two beads on a cosmic string, the interacting galaxy pair known as Keenan’s System in the constellation Ursa Major is the subject of this spectacular image! The lower of the two is catalogued as NGC 5216 while the more northern one is NGC 5218. Both are collectively known as Arp 104 due to their inclusion in Halton Arp’s atlas of peculiar galaxies.

Gravitational interaction is now known to be a common part of galaxy evolution and there are many visually distinguishing signs. In NGC 5216, a series of concentric tidal shells can be seen enveloping the bright core, whereas normal elliptical galaxies display a simple and undisturbed appearance. In addition to the shells, an impressive blue tidal plume with a curved morphology is connected to the nucleus and is likely to be the remains of a spiral galaxy that might have previously merged with it. NGC 5218 to the north is a heavily deformed spiral galaxy with a chaotic structure, which also consists of a diffuse tidal plume. The vast array of tidal features suggests that the whole galaxy system is the result of interactions and mergers between multiple members of a small group of galaxies.

Arguably the most striking of the retinue of peculiar features and structures associated with Keenan’s System is the huge tidal bridge connecting the two galaxies together. Tidal bridges are rare with very few examples of connected systems, other interacting galaxies belonging to this category include Arp 295 and NGC 4410. At an approximate distance of 137 million light years, the extremely smooth tidal bridge has an estimated length of 124,000 light years! This is coupled with a very narrow width of 7000 light years. The origin of tidal bridges are stars of the outer parts of galaxies being tidally stripped and reconfigured into a narrow structure. The majority of the blue stars in the bridge originate from NGC 5218. This galaxy system is named after Philip Keenan who first discovered this tidal bridge photographically in 1935.

Detailed radio observations have detected a massive amount of neutral gas (HI) associated with NGC 5218, the raw intergalactic fuel for star formation. Unsurprisingly NGC 5218 is undergoing recent starburst activity with many ionized HII regions revealed in Ha images taken with professional observatories. The neutral gas associated with NGC 5218 is also heavily concentrated in the northern plume and is significantly larger than its optical counterpart. It is highly probable that NGC 5218 experienced a merger between two small galaxies prior to its interaction with NGC 5216. The likely fate of this system is both galaxies will merge into a single larger elliptical galaxy.

With an almost lifelong obsession with the universe, his interests include deep sky objects, particularly planetary nebulae, galaxies and exotic stellar ejecta. A never ending curiosity compels him to continue to explore the vast realm of obscure deep sky objects. Contact Sakib.